The present invention relates to an exhaust gas analyzing system.
Currently, a CVS method (Constant Volume Sampling) is widely used as a sampling method to measure mass of components in gas exhausted from an engine of an automobile. A possibility of insufficient accuracy is pointed out in measuring exhaust gas of a ULEV (Ultra Low Emission Vehicle), a SULEV (Super Ultra Low Emission Vehicle), and like when the CVS method is used.
A substitute for the above CVS method is a mini-diluter method. In the mini-diluter method, a portion of the exhaust gas is sampled instead of diluting the entire quantity of exhaust gas from the engine. The sampled exhaust gas is diluted at a certain dilution ratio, the diluted sample gas is gathered in a sample bag by an amount proportional to a flow rate of the exhaust gas from the engine, and the diluted sample gas in the sample bag is analyzed.
FIG. 3 schematically shows an example of an exhaust gas analyzing system for which the mini-diluter method is used. Reference numeral 1 represents an engine of an automobile, reference numeral 2 represents an exhaust gas flow path connected to an exhaust pipe connected to the engine 1, and reference numeral 3 represents a flowmeter (digital flowmeter, for example) for measuring a flow rate of the entire exhaust gas G flowing through the exhaust gas flow path 2. Reference numeral 4 represents a sampling flow path that is connected to the exhaust gas flow path 2 at a point 5 downstream from the flowmeter 3. A portion of the exhaust gas G, which is sample gas S, flows through the sampling flow path 4.
Reference numeral 6 represents a mini-diluter which is coupled to the sampling flow path 4. Reference numeral 4A represents a sampling flow path in the mini-diluter 6 in which a CFV (critical flow venturi) 7 for defining flow rate of the sample gas S flowing through the sampling flow path 4A and a suction pump 8 are provided. Reference numeral 9 represents a dilution gas flow path provided in parallel with the sampling flow path 4A. A pressure controller 10 and a CFV 11 is provided in the dilution flow path for defining a flow rate of the dilution gas D. A downstream side of the CFV 11 is connected to the CFV 7 by the sampling flow path 4A at a point 12 which is between the CFV 7 and the pump 8. The pressure controller 10 equalizes pressure on an inlet side of the CFV 7 of the flow path 4A with pressure on an inlet side of the CFV 11 of the dilution gas flow path 9. A cylinder 13 containing dilution gas (e.g., nitrogen gas) is provided upstream of the pressure controller 10 (more specifically, outside the mini-diluter 6).
Sampling flow path 4A includes a sample bag 16 which is provided downstream from the suction pump 8. A mass-flow controller 14 (MFC) includes a flow rate measuring portion and a flow rate control valve. The mass-flow controller 14 measures and controls the flow rate via a three-way solenoid valve 15 as a selector valve. Reference numeral 17 represents an overflow flow path, and the overflow path 17 is connected to a point 18 between the suction pump 8 of the sampling flow path 4A and the mass-flow controller 14.
Reference numeral 19 represents a gas analyzing portion provided in a rear stage of the mini-diluter 6, and a plurality of gas analyzers 19a to 19n, for example, are provided in parallel with each other in a flow path 20. The flow path 20 is connected to the three-way solenoid valve 15. Exemplary gas analyzers 19a to 19n are NDIR (non-dispersive infrared analyzer) for measuring CO and CO2, CLD (chemiluminescent analyzer) for measuring NOx, FID (flame ionization detector) for measuring THC (total hydrocarbon), and the like.
Furthermore, reference numeral 21 represents an arithmetic controller having a personal computer, for example. The arithmetic controller performs computations based on output signals from the flowmeter 3, mass-flow controller 14, and gas analyzing portion 19 and controls the entire exhaust gas analyzing system based on a result of the computations.
For the exhaust gas analyzing system having the above structure and for which the mini-diluter method is used, the exhaust gas analysis is carried out as follows. Flow rate of the exhaust gas G from the engine 1 is measured by the flowmeter 3 and output from the flowmeter 3 is input into the arithmetic controller 21. Because the suction pump 8 in the mini-diluter 6 is operating, a portion of the exhaust gas G, wherein a flow rate has been measured, is taken in the sampling flow path 4 as the sample gas S. The sample gas S flows through the flow path 4A of the mini-diluter 6 toward the suction pump 8. By operation of the suction pump 8, the dilution gas D flows through the dilution gas flow path 9 provided in parallel with the flow path 4A.
In this case, because the dilution gas flow path 9 is provided with the pressure controller 10 which equalizes the pressure on the inlet side of the CFV 7 of the flow path 4A with the pressure on the inlet side of the CFV 11 of the dilution gas flow path 9 and because the flow path 4A and the dilution gas flow path 9 are respectively provided with the CFVs 7 and 11 for defining the flow rates of the gas S and D flowing through the flow paths, 4A and 9, ways of changing flow rates of the gas S and D flowing through both flow paths 4A and 9 are equalized with each other and a ratio between the flow rates is always constant. The gas flows S and D merge with each other at a confluence 12, and the sample gas S is diluted with the dilution gas D to a certain consistency.
The diluted sample gas S flows through the suction pump 8 to a downstream side of the pump 8, and a portion of the gas S flows toward the three-way solenoid valve 15. Flow rate of the portion of the gas S flowing towards the three-way solenoid valve is set by the mass-flow controller 14 provided in the flow path 4A. Because the three-way solenoid valve 15 allows the mass-flow controller 14 and the sample bag 16 to communicate with each other when the power is turned off, the diluted sample gas S which has passed through the mass-flow controller 14 is gathered in the sample bag 16. The remainder of the diluted sample gas S is exhausted through the overflow flow path 17.
An opening degree of the flow rate control valve of the mass-flow controller 14 is controlled actively such that the flow rate of the diluted sample gas S passing through the mass-flow controller 14 is proportional to a flow rate of the exhaust gas G flowing through the exhaust gas flow path 2. More specifically, because the flow rate of the exhaust gas is measured by the flowmeter 3 and the result of the measurement is input into the arithmetic controller 21 as described previously, the arithmetic controller 21 sends a control command to set the opening degree of the flow rate control valve of the mass-flow controller 14 at a predetermined value. Thus, the mass-flow controller 14 allows the sample gas S to flow at a proportional flow rate to the flow rate of the exhaust gas G.
When the predetermined sampling ends, power to the three-way solenoid valve 15 is turned on, the sample bag 16 and the flow path 20 communicate with each other, the diluted sample gas S taken into the sample bag 16 is supplied to the gas analyzing portion 19, and concentrations of components to be measured contained in the diluted sample gas S (e.g., CO, CO2, NOx, and THC) are respectively measured by NDIR, CLD, FID, and the like.
In this case, mass Mx of a component X before dilution is given by the following expression (1).
Mx=Cxbagxc3x97Vexxc3x97Rxc3x97xcfx81xxe2x80x83xe2x80x83(1)
Where Cxbag represents a measured concentration of the component X in the bag, Vex represents total volume of the exhaust gas, Rd represents dilution rate, xcfx81x density of the component X.
The mass Mx of component X in the exhaust gas G before dilution can be easily obtained because the measured concentration Cxbag of the component X in the bag, the total flow rate Vex of the exhaust gas, the dilution rate Rd, and the density xcfx81x of the component X are respectively known. According to the exhaust gas analyzing system for which the mini-diluter method is used, the mass of the low-concentration exhaust gas component can be accurately measured.
However, in the exhaust gas analyzing system for which the mini-diluter method is used, it is essential to measure the flow rate of the exhaust gas G in real time, and a large error may be incorporated into the finally obtained mass of the component in the exhaust gas because it depends on the measurement error of the flow rate.
The present invention has been accomplished with the above circumstances in view, and it is an object of the present invention to provide an exhaust gas analyzer in which measurement accuracy of the exhaust gas analyzing system utilizing a mini-diluter method can be evaluated by the system itself.
To achieve the above object, in accordance with the present invention, an exhaust gas analyzing system is provided. The exhaust gas analyzing system comprises a sampling flow path connected to an exhaust gas flow path through which gas exhausted from an engine flows to sample a part of the exhaust gas. The sampled exhaust gas is diluted with dilution gas introduced through a dilution gas flow path connected in parallel to the sampling flow path. A portion of the diluted exhaust gas is stored in a sample bag. The diluted exhaust gas in the sample bag is analyzed in a gas analyzing portion, wherein flow rate of the exhaust gas is measured, trace gas with a known concentration is introduced into the exhaust gas flow path on an upstream side from a connecting point between the exhaust gas flow path and the sampling flow path while monitoring flow rate of the trace gas. The diluted trace gas is analyzed in the gas analyzing portion, and total mass of the trace gas calculated from a result of the analysis is compared with total mass of the introduced trace gas to evaluate measurement itself.
In the exhaust gas analyzing system, the total mass of the trace gas introduced into the exhaust gas flow is known. It is possible to evaluate accuracy of the measurement itself by comparing the total mass of the measured and calculated trace gas (using the mini-diluter method) with the above known total mass. As a result, reliability of the exhaust gas analyzing system for which the mini-diluter method is used and which includes measurement of the flow rate of the exhaust gas is improved.